Measurement of phase transformations in steel using electromagnetic sensors

An electromagnetic (EM) sensor, capable of detecting the formation of ferromagnetic ferrite from paramagnetic austenite below the Curie temperature, has been developed and assessed. The long term aim of this work is to develop a method for monitoring microstructure online during strip steel processing. In the present paper, the initial results of variation in trans-impedance with microstructure obtained for three different types of steel with varying carbon contents are discussed. It was found that the EM sensor can successfully detect the formation of ferrite below the Curie temperature, but trans-impedance values are affected by the presence of a decarburised ferrite ring that forms around the specimens tested in a furnace. It was also found that the trans-impedance value is monotonically (non-linearly) related to ferrite volume fraction, and depends on the morphology and distribution of the ferromagnetic phase and, hence, is influenced by the prior austenite grain size.